1. Technical Field
The present invention relates to a laser printer of printing rasterized data of high quality of picture using a graphic descriptive interpretation (GDI), and particularly to a laser printer capable of adjusting the difference of a resolution density (i.e., a number of dots per inch "DPI") of data received from a host computer and a resolution density of a printer engine.
2. Related Art
Generally, a graphic descriptive interpretation (GDI) method requires a personal computer (PC) to rasterize image data for printing purposes and then transmit the rasterized image data to a printer for printing according to the rasterized image data. In contrast to most contemporary printing methods which require many memory devices, highly efficient processors, font storing memories and video controllers including data decoding program, etc., the GDI method requires the personal computer (PC) to process image data and then transmit the image data to the video controller of the printer which serves only as a buffer for subsequent printing purposes.
The rasterized image processing at the personal computer (PC) is a technique which forms an image in terms of a graphic mode. First, an image is regarded as a collection of small dots, and is formed by the shading and coloring of each dot. Using this technique, as everything else is displayed on the screen by means of dots, the density resolution is determined according to the size and number of dots per unit length. For example, when rasterized data having a density resolution of 300 DPI (Dot Per Inch) in the personal computer using the GDI technique is transmitted to the printer engine of a printer, the quality of image is poor, as the printer engine prints image data having 600 DPI if the printer engine has a density resolution above 600 DPI. That is, only half of image data is printed in a horizontal direction (i.e., scanning direction), and the other half of image data is printed in a vertical direction (i.e., paper driving direction). As a result, only the area of one fourth (1/4) of image data is printed. In order to solve this problem, the printing of image data in the horizontal direction is performed by reducing a video frequency to a half (1/2), and the printing of image data in the vertical direction is performed at every other line. This way, the printing area can be restored to the former state. However, the resolution density of an image is low due to the difference of a dot density (i.e., dots per inch) of image data for printing and a dot density of the printer engine.
Contemporary efforts for laser type printer to print image data in different resolution density are disclosed in U.S. Pat. No. 5,123,082 for Image Information Recorder Having A Resolution Density Transformation Device issued to Shimada, U.S. Pat. No. 5,239,313 for Continuously Variable Resolution Laser Printer issued to Marko et al., U.S. Pat. No. 5,289,564 for Image Recording Apparatus For Providing High Quality Image issued to Morimoto et al., and U.S. Pat. No. 5,548,690 for Printing Apparatus issued to Shimada. For example, Shimada '082 discloses a laser beam printer with a resolution density transformation device for transforming image data of one resolution density into another resolution density that is used by the laser beam printer. Similarly, Morimoto et al. '564 discloses a laser beam printer having conversion capability for converting image data received from a host computer from one resolution density into another resolution density that is used by the laser beam printer. Likewise, Shimada '690 uses an interpolation technique to control the resolution density of image data for printing purposes. Marko '313 discloses a laser beam printer having continuously selectable printing resolutions by changing, however, various combinations of laser spot size, paper motion speed, video data rate, and scanner speed. As I have observed, however, a simpler technique for overcoming the difference between the resolution density of image data received from a host computer and the resolution density used by the laser beam printer for high quality image is still needed.